I'm not quite sure how using a character with a high MPR to forge is better than say one with 1 MPR. The larger character gets a bonus, but the smaller gets more regenerated BA and the purchasable BA costs next to nothing. I guess I just want someone to explain to me why a larger character is better and by how much.

The NW forged per ba is not always less than what it costs to buy ba but the amount of money you will make from it is. That is what the forging efficiency is based on. How much NW forged compared with the price of the ba.